When an oil becomes oxidized, it first develops hydroperoxides. Hydroperoxides have no flavor or odor. However, they break down rapidly to fform aldehydes, which have a strong, disagreeable flavor and odor. The overall flavor defect is called oxidative rancidity.
It is well known to subject fats and oils to final deodorization in the process of preparing them for commercial use. Deodorization is the process whereby odors and flavors of fats and oils are removed, usually by blowing steam through hot oil at 200.degree.-275.degree. C. (392.degree.-527.degree. F.) under low pressure (3-10 Torr).
It is a desirable practice in deodorization, as a final step, to add citric acid to the oil. It is usually added as a water solution under vacuum, so that the water is evaporated off. The citric acid functions as a metal scavenger, especially for traces of copper and iron which act as pro-oxidants for oil. If citric acid is not added, the oil can revert and oxidize more rapidly.
Hydrolytic rancidity is the reaction of triglycerides with water, to produce glycerin and free fatty acids. The reaction can be exemplified by the following equation: EQU C.sub.3 H.sub.5 (OOCR).sub.3 +3HOH=C.sub.3 H.sub.5 (OH).sub.3 +3HOOCR
Prior to deodorization, fats and oils are usually subjected to alkali refining. Alkali refining is effective to achieve almost complete removal of free fatty acids. The free fatty acid content is determined by titration of a sample with a standard solution of sodium hydroxide (AOCS method Ca5a-40).
However, after refining, oil is often held in storage tanks. Water vapor can get into these tanks through breather pipes and condense on cooling. The free fatty acid content can increase again because of the presence of such water. Deodorization, the final process before the fat or oil is packaged, lowers free fatty acid and water to about 0.05% or less. However, moisture in pipe lines, feed tanks, beading towers, and flaking rolls as well as tank wagons and cars, can again cause subsequent increase in moisture, in turn causing again an increase in free fatty acid content.
Lauric fats and oils, such as coconut oil and palm kernel oil, are particularly subject to hydrolytic rancidity. Hydrolysis of coconut oil or palm kernel oil, even to a small extent, liberates short-chain fatty acids, which are highly flavored and have a very disagreeable soapy flavor.
The problem with the presence of citric acid in lauric fats and oils, when accompanied by moisture, is that the citric acid, in effect, can function as a catalyst in the hydrolysis reaction, causing an increase in percent free fatty acid. Work carried out in connection with the present invention has demonstrated that the rancidity occurred at a rate proportional to the level of citric acid present. The most stable fractionated palm kernel oil samples obtained other than by the present invention were those containing no citric acid and low levels of water (below 0.01%).
As a result, citric acid is normally added only to domestic fats and oils, and not lauric fats and oils.
Lecithin is a well-known additive to fats and oils for a number of purposes, mostly involving emulsification or viscosity reduction. For instance, lecithin is used to reduce the viscosity of confectionery coatings, or as an emulsifier in such products as ice cream, bread, icings, paints, cosmetics, printing inks, and the like. Also, it is known to have a synergistic action with phenolic antioxidants and may be used for this purpose. In such uses, the amount employed can be characterized as an emulsifying amount or antioxidant amount. By way of example, in confectionery products a reduction in viscosity is achieved by the addition of about 0.1-0.4% lecithin, based on the weight of the coating.
It is also known to package lauric hard butters with about 0.1% lecithin added as a moisture scavenger. This is said to reduce the potential of developing hydrolytic rancidity in a stored, deodorized fat. This disclosure can be found in the book "Food Oils and Their Uses", Second Edition, Theodore J. Weiss, The Avi Publishing Co., Inc. copyright 1983 (page 289).
Prior patent to Black, U.S. Pat. No. 2,494,114, describes the stabilization of fatty materials by the addition of an anti-oxidant known as "NDGA" (dihydroguaiaretic acid). The patent suggests that improved stabilization against rancidity produced by oxidation can be achieved by the use of lecithin or citric acid in combination with NDGA. One sample reported in the patent contains all three ingredients, NDGA, 0.002% citric acid, and 0.03% lecithin. However, the data given with regard to rancidity produced by oxidation (rancid in 80 days) was no better than that obtained by the use of NDGA and citric acid alone (also 80 days). The patent makes no mention of hydrolytic rancidity, nor the particular problems associated with lauric fats and oils.
Applicants know of no disclosure that teaches treating lauric fats or oils with citric acid, to sequester traces of copper and iron, and then further adding lecithin for the purpose of achieving substantially complete resistance to not only oxidative rancidity but also hydrolytic rancidity as well.